I dont know the specifics of this exact model, but normally when you turn on the key you send voltage thru a wire to the the float on top of the gas tank. THe float has a variable resistor that changes with float level. The gas gauge is basically a volt meter reading the voltage drop across the resistor inside the gas tank. If the wire breaks or the resistor breaks, you probably read zero.... or the gauge itself could be bad. If it was me i would crawl under the truck and look by the gas tank for a wire running to the top of the tank. If you bare a section of the wire and short it to ground (the frame) the gas gauge should go to max up or down. When you pull the wire off ground, and touch nothing, it will go the other way. I assume with no connection it goes to zero and when shorted to ground it is "full". You might also look in your operators manual and see which fuse controls the gas gauge and check it to make sure you have voltage TO the tank. If all this failes, then i assume the gauge is bad.